Uses :singles as the concurrency model

Fixes #96

Author: ph

lib/logstash/outputs/s3.rb

@@ -79,7 +79,7 @@ class LogStash::Outputs::S3 < LogStash::Outputs::Base
   config_name "s3"
   default :codec, 'line'
 
-  concurrency :shared
+  concurrency :single
 
   # S3 bucket
   config :bucket, :validate => :string